A subscriber's telephone line is powered by a continuous voltage generator in series with which are connected other voltage signal generators such as the speech band call signals, ringing signals and subscriber charge indicating signals.
To the terminals of the line are connected in mutual parallel both the call circuit and the ringing circuit of the subscriber's telephone equipment and other devices, if any, inside or outside the equipment, such as, e.g., a progressive counting indicator for the current telephone connection charge which is calculated by the dedicated exchange mechanisms or organs.
The subscriber's charge indicators are driven as mentioned above by the telephone exchange through the same telephone line by means of dedicated control signals termed normally "charge signals."
These control signals cannot be provided as signals having a pulsed wave form, because in this case, the rising and falling fronts of the pulses would create harmonics such as to disturb not only the line subscriber but also the subscribers of the neighboring lines.
In practice, alternating voltage signals are used having a limited duration in time (approximately 100 msec) and frequency (12 or 16 kHz) much greater than that of the call telephone signals (0.3+3.4 kHz) with relatively long signal level rise and fall times (approximately 10 ms).
There are electromechanical solutions in which the "charge signals" are formed and sent directly on line by means of exchange devices comprising a resonant circuit coupled inductively through a transformer to the subscriber's telephone line and activated for certain time periods. The behavior of the signal level increase and decrease is exponential.
The solution seen above is also relatively costly and requires tuning the resonant circuit exactly to the desired frequency.
In U.S. Pat. Nos. 4,638,122 and 4,720,852 of this applicant, there are indicated solutions better suited to the requirements of modern electronic telephone systems and describing control signal generating telephone circuits for the subscriber's charge indicators and suited to being integrated monolithically and constituting together with the subscriber's line speech circuit an interface between the subscriber's telephone line and the exchange control organs.
In the former of the above-mentioned United States patents is described a telephone circuit which is a monolithically integrable generator of control signals for the charge indicators and is coupled to an alternating voltage signal generator having predetermined amplitude and frequency constant in time and comprises a generator of voltage signals spaced in time and having a pulsed trapezoid wave-form. Therein a multiplying circuit finds the product of the signals supplied by the two generators to supply a signal which is sent to the subscriber's line speech circuit and added to the call signals.
It also comprises circuit means designed to take an image signal of the whole signal adapted to the line, a high-pass filter to eliminate the call signal components, a rectifier circuit for rectifying the filtered signal, and a comparator for comparing the rectified signal with the reference signal. If the amplitude of the rectified signal is greater than the amplitude of the reference signal the comparator generates a signal capable of causing stopping of the level increase of the pulsed trapezoid signals to regulate amplitude.
In the latter of the above-mentioned United States patents another telephone circuit is described. The telephone circuit is a monolithically integrable generator of control signals for the charge indicators and is coupled to an alternating voltage signal generator having predetermined amplitude and frequency constant in time and comprises a generator of voltage signals spaced in time and having a pulsed trapezoid wave-form. In the latter also, a multiplying circuit finds the product of the signals supplied by the two generators to supply a signal which is added to the call signals and sent to the subscriber's line speech circuit (VC).
The generator of signals having a pulsed trapezoid waveform comprises a capacitor which is charged and discharged by means of a voltage-current transducer of the non-linear type driven by the exchange control organs. The voltage at the ends of this capacitor constitutes the signal supplied by the generator of signals having trapezoid wave-form.
The transducer supplies a current proportional to the voltage applied to its input terminals for values of the latter between two predetermined threshold values opposite in sign and equal in absolute value and beyond which the current supplied remains constant as the input voltage changes.
The purpose of the present invention is to provide a monolithically integrable control signal generating telephone circuit for subscriber's charge indicators and designed to constitute together with the subscriber's line speech circuit, an interface between the subscriber's telephone line and exchange control organs which would be economically advantageous with respect to the devices of the prior art.
This purpose is achieved by the "charge signal" generating telephone circuit defined and characterized in the claims which conclude the present invention.
The characteristics and advantages of the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings.